This invention relates to an improved method for transporting particles from one surface to another using electrode plates having a direct current potential difference between them. The invention also relates to methods and articles for minimizing static buildup on particles and surfaces.
The placement of particles, such as electrically conductive solder, on contact pads is critical to the adoption of array style semiconductor packages such as ball grid arrays (BGA). Such placement is also critical in the attachment of integrated circuits (IC) to packages or printed circuit boards through flip chip processes. Recent attempts have been made to improve, for example, solder ball interconnects, such that more reliable and/or less costly solder connections are made in electronic applications. Despite these efforts, there are still problems associated with the handling and transfer of particles, primarily conductive particles such as solder balls to form solder bumps, on the contact pads of electronic devices. There is a need for further improvements, particularly with regard to the efficiency, precision, and robustness of the process.
The present invention is a method for transferring particles from an electrode plate to tacky areas present on a substrate comprising:
a) placing a substrate having both tacky and non-tacky areas between first and second electrode plates, the substrate and electrode plates arranged substantially horizontally and stacked substantially vertically, wherein the first electrode plate
(i) lies below the substrate,
(ii) has a surface which faces tacky and non-tacky areas on the substrate, and
(iii) is spaced from the substrate and the second electrode plate;
b) applying particles over the surface of the first electrode plate; and
c) applying a direct current potential between the first and second electrode plates for a time T1, establishing a polarity on the first electrode and thereby causing the particles to be charged and be propelled toward the second electrode plate, resulting in at least a portion of the charged particles becoming adhered to tacky areas on the substrate.
Another embodiment of the invention having additional step(s) includes as step d) changing the direct current potential on the first electrode plate for a time T2 after step c) to cause at least some of the particles to leave non-tacky areas of the substrate, be propelled against the first electrode, again be charged and be propelled toward the second electrode. Still other embodiments include repeating steps c) and d) for a number, N, of cycles as step e), eliminating the direct current potential between the electrode plates and removing particles from the non-tacky areas as steps f) and g), and placing a non-conductive shield between the substrate and the first electrode plate as step h).
In another embodiment, the invention is a method for mounting particles on a substrate having both tacky and non-tacky areas thereon, wherein a direct current potential between first and second electrode plates is used in the method and in which the particles are first applied to a substrate having tacky and non-tacky areas, which substrate is placed over the first electrode.
In further embodiments, the tacky areas are heated to improve adhesion and centering of the particles in the tacky areas. The invention also comprises apparatuses for practicing the above methods. The invention also comprises an article having a substrate or surface with tacky and non-tacky areas that has an electrically conductive surface adjacent to the tacky and non-tacky areas to dissipate electrostatic charges and a method for changing the tacky and non-tacky areas.